[core] Only snapshot the user config when esphome config --no-defaults asks for it (#18113)

tests/unit_tests/test_main.py

@@ -6362,6 +6362,26 @@ def test_run_esphome_skip_external_update_per_command(
     assert mock_read.call_args.kwargs["skip_external_update"] is expected_skip
 
 
+@pytest.mark.parametrize(
+    ("argv_extra", "expected"),
+    [(["--no-defaults"], True), ([], False)],
+)
+def test_run_esphome_snapshot_user_config_only_for_no_defaults(
+    tmp_path: Path, argv_extra: list[str], expected: bool
+) -> None:
+    """read_config is invoked with snapshot_user_config=True only when the
+    config command is run with --no-defaults; otherwise the expensive deep
+    copy is skipped."""
+    yaml_file = tmp_path / "device.yaml"
+    yaml_file.write_text("esphome:\n  name: test\n")
+
+    with patch("esphome.config.read_config", return_value=None) as mock_read:
+        run_esphome(["esphome", "config", str(yaml_file), *argv_extra])
+
+    mock_read.assert_called_once()
+    assert mock_read.call_args.kwargs["snapshot_user_config"] is expected
+
+
 def test_get_configured_xtal_freq_reads_sdkconfig(tmp_path: Path) -> None:
     """Test reading XTAL_FREQ from sdkconfig."""
     CORE.name = "test-device"

tests/unit_tests/test_substitutions.py

@@ -370,7 +370,7 @@ def test_validate_config_captures_user_config_snapshot(tmp_path: Path) -> None:
     """
     test_config = _get_test_minimal_valid_config(tmp_path)
 
-    result = config_module.validate_config(test_config, None)
+    result = config_module.validate_config(test_config, None, snapshot_user_config=True)
 
     # Snapshot is populated.
     assert result.user_config is not None
@@ -393,7 +393,7 @@ def test_validate_config_user_config_snapshot_is_deep_copy(tmp_path: Path) -> No
     """
     test_config = _get_test_minimal_valid_config(tmp_path)
 
-    result = config_module.validate_config(test_config, None)
+    result = config_module.validate_config(test_config, None, snapshot_user_config=True)
 
     assert result.user_config is not None
     # preload_core_config injected build_path onto the validated config.
@@ -404,6 +404,32 @@ def test_validate_config_user_config_snapshot_is_deep_copy(tmp_path: Path) -> No
     assert result["esphome"] is not result.user_config["esphome"]
 
 
+def test_validate_config_snapshot_without_substitutions(tmp_path: Path) -> None:
+    """The snapshot works for configs that have no substitutions block."""
+    test_config = _get_test_minimal_valid_config(tmp_path)
+    del test_config[CONF_SUBSTITUTIONS]
+
+    result = config_module.validate_config(test_config, None, snapshot_user_config=True)
+
+    assert result.user_config is not None
+    assert CONF_SUBSTITUTIONS not in result.user_config
+    assert result.user_config["esphome"] == {"name": "test_device"}
+
+
+def test_validate_config_skips_user_config_snapshot_by_default(
+    tmp_path: Path,
+) -> None:
+    """Without ``snapshot_user_config`` the deep copy is skipped entirely;
+    only ``esphome config --no-defaults`` needs the snapshot and the copy is
+    too expensive to take on every load.
+    """
+    test_config = _get_test_minimal_valid_config(tmp_path)
+
+    result = config_module.validate_config(test_config, None)
+
+    assert result.user_config is None
+
+
 def test_merge_config_preserves_ordered_dict() -> None:
     """Test that merge_config preserves OrderedDict type.